Ukraine has agreed to pump enough natural gas into its storage facilities to maintain pressure in the system so Russian gas can reach Europe in winter, Russian Energy Minister Aleksandr Novak has said.

Around 40 percent of Russian gas exports to the European Union traverse Ukraine.

Novak said on June 3 that Ukraine had agreed to fill the storage facilities after talks in Brussels between Russia, Ukraine, and the European Commission June 2.

A source close to the talks told Reuters that Moscow and Kyiv had agreed to the outlines of a protocol on the gas trade, and that ministers would meet later this month to try to resolve the key outstanding disagreements.

Outstanding issues include how long the protocol would last, the formula applied to provide discounted gas prices for Ukraine, and the fees charged by Ukraine for the transit of gas over its territory.

Novak told reporters in Vienna: "They agreed on some positions of the protocol, including on pumping no less than 19 bcm into the storages before the forthcoming winter."

There was no immediate comment from Ukraine, which has said it needs from 14 billion to 19 billion cubic meters (bcm) in storage to keep Russian gas flowing to Europe.

"We agreed that the protocol will include the financial assistance of the European Commission for gas purchases by Ukraine in case such funds are needed," he said.

"The European side would assume responsibility to provide financial assistance, because, you understand, that [in Ukraine] there
is a problem now with money to pay for gas to inject into underground storage facilities," he told the BBC.